Question

A 5W source emits monochromatic light of wavelength 5000 Å. When 0.5 m away it liberates photoelectrons from a photosensitive metallic surface. When the source is moved to a distance of 1.0 m, the number of photoelectrons liberated will be reduced by a factor of –

• A. 4
• B. 8
• C. 16
• D. 2

Answer 1

Answer: (A)

4

Answer 2

P = $\frac { \mathrm { nE } } { \mathrm { t } }$

5 = $\frac { \mathrm { n } } { \mathrm { t } } \times \frac { 20 \times 10 ^ { - 26 } } { 5000 \times 10 ^ { - 10 } }$

$\frac { \mathrm { n } } { \mathrm { t } } = \frac { 5000 \times 5 } { 20 } \times 10 ^ { 16 }$ photon/s

$\frac { \mathrm { n } _ { \mathrm { e } _ { 1 } } } { \mathrm { n } _ { \mathrm { e } _ { 2 } } } = \left( \frac { \mathrm { r } _ { 2 } } { \mathrm { r } _ { 1 } } \right) ^ { 2 } = \left( \frac { 1 } { 0.5 } \right) ^ { 2 } = 4$ $\mathrm { n } _ { \mathrm { e } _ { 2 } } = \frac { \mathrm { n } _ { \mathrm { e } _ { 1 } } } { 4 }$